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A unified constitutive model for unsaturated soils is presented in a critical state framework using the concepts of effective stress and bounding surface plasticity theory. Consideration is given to the effects of unsaturation and particle crushing in the definition of the critical state. A simple isotropic elastic rule is adopted. A loading surface and a bounding surface of the same shape are defined using simple and versatile functions. The bounding surface and elastic rules lead to the existence of a limiting isotropic compression line, towards which the stress trajectories of all isotropic compression load paths approach. A non‐associated flow rule of the same general form is assumed for all soil types. Isotropic hardening/softening occurs due to changes in plastic volumetric strains as well as suction for some unsaturated soils, enabling the phenomenon of volumetric collapse upon wetting to be accounted for. The model is used to simulate the stress–strain behaviour observed in unsaturated speswhite kaolin subjected to three triaxial test load paths. The fit between simulation and experiment is improved compared to that of other constitutive models developed using conventional Cam‐Clay‐based plasticity theory and calibrated using the same set of data. Also, the model is used to simulate to a high degree of accuracy the stress–strain behaviour observed in unsaturated Kurnell sand subjected to two triaxial test load paths and the oedometric compression load path. For oedometric compression theoretical simulations indicate that the suction was not sufficiently large to cause samples to separate from the confining ring. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
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The shear modulus at very small strains (less than 0.001%) is an important parameter in the design of geotechnical structures subjected to static and cyclic loadings. Although numerous soil models are available for predicting shear modulus of saturated and dry soils, only a few ones can predict shear stiffness at very small strains of unsaturated soils correctly. In this study, a few unsaturated soil models are evaluated critically and compared with a newly developed model. This newly proposed model is verified by using measured shear modulus at very small strains for three different low plasticity fine grained soils available in the literature. It is found that this new model can predict shear modulus at very small strain resulting from an increase and a decrease in mean net stress at constant matric suction for low plasticity fine grained soils. Moreover, this model is able to give a reasonably good prediction on shear stiffness at very small strain during wetting of a collapsible unsaturated soil. In addition, the newly proposed model is illustrated to capture a consistent trend with experimental data of shear stiffness at very small strain for non-collapsible soils obtained during drying–wetting cycles. This evaluation revealed that the newly proposed model has better predictive capabilities than some earlier formulations of the same simplicity. In addition, the proposed model with fewer parameters has similar predictive capability as compared with a more complex model. 相似文献
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An adaptive substepping explicit integration scheme is developed for a porosity‐dependent hydro‐mechanical model for unsaturated soils. The model is referred to as the modified σ –Θ model in this paper, which features the employment of the subloading surface plasticity and the stress–saturation approach. On numerical aspects, convex/nonconvex subloading surfaces in the σ –Θ space may result in incorrect loading–unloading decisions during the integration. A new loading–unloading decision method is developed here to solve the problem and then embedded into the explicit integration scheme for the modified σ –Θ model. In addition, to enhance the accuracy of the explicit integration, local errors from both hydraulic and mechanical components are included in the error control for each substep. A drift correction method is also developed to ensure the state point lies on the subloading surface in the σ –Θ space within a set error level. The performance of the loading–unloading decision method for the modified σ –Θ model is discussed through comparing it with the conventional loading–unloading decision method. The importance of involving the hydraulic component in the error control is also demonstrated. The accuracy and efficiency of the proposed adaptive substepping explicit integration scheme for the modified p–Θ model are also studied via several numerical examples. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
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The accurate simulation of complex dynamic phenomena requires the availability of advanced constitutive models capable of simulating a wide range of features of soil behaviour under cyclic loading. One possible strategy is to improve the capabilities of existing bounding surface plasticity models, as this framework is characterised by its modularity and flexibility. As a result, specific components of the formulation of this type of model may be adjusted to improve the reproduction of any aspect of soil behaviour deemed essential to the problem being analysed. In this paper, a series of computational studies are performed in order to establish the impact of expanding a bounding surface plasticity model for sands on its modelling capabilities and to suggest ways of mitigating the associated increase in complexity. Changes to three distinct aspects of the selected constitutive model are examined: the shape of the Critical State Line in p′ − e space, the expression used for calculating the hardening modulus and the form of the yield surface. It is shown that the introduced changes have the potential to increase significantly the ability to control how certain aspects of soil response, such as degradation of stiffness and flow liquefaction with limited deformation, are reproduced by the model. Moreover, this paper presents a systematic approach to the expansion of this type of constitutive model, establishing how alterations to the formulation of a model may be assessed in terms of improved accuracy and potential benefits. 相似文献
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The coupled hydro‐mechanical state in soils coming from consolidation/subsidence processes and undergoing plasticity phenomena is here evaluated by means of the subloading surface model. The most important feature of this theory is the abolition of the distinction between the elastic and plastic domains, as it happens in the conventional elastoplastic models. This means that plastic deformations are generated whenever there is a change in stress and a smoother elastoplastic transition is produced. The plasticity algorithm has been implemented in the PLASCON3D FE code, coupling hydro‐thermo‐mechanical fields within a saturated (locally partially saturated) porous medium subjected to external loads and water/gas withdrawals from deep layers (aquifers/reservoirs). The 3D model has been first calibrated and validated against examples taken from the literature, and then subsidence analyses at regional scales due to gas extractions have been developed to predict the evolution of settlements and pore pressure in soils for long‐term scenarios. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
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天然土体的初始各向异性通常可对其后继循环特性产生显著影响。现有考虑循环载荷作用的土体弹塑性模型,往往采用类似修正剑桥模型的椭圆形屈服面,已有研究表明,该椭圆形屈服面因其拉伸弹性区域偏大,针对天然K0固结状态的土体,其计算精度较差。基于新近提出的广义各向同性硬化准则,在边界面方程中引入初始各向异性张量,并采用空间滑动面破坏准则（SMP）的变换应力法,建立了能考虑饱和黏土初始各向异性的循环边界面塑性模型。分别针对等压和偏压固结的饱和黏土静、动三轴试验进行模拟,结果表明,该模型能合理反映土体的初始各向异性及其后继循环动力特性。 相似文献
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修正了传统隐式回映算法,建立了适用于饱和黏土循环动力分析的边界面塑性模型的完全隐式积分格式。该模型基于无弹性域概念和临界状态理论,采用各向同性、运动硬化准则、旋转的边界面,并引入表征土体结构损伤和重塑程度的损伤变量以反映循环载荷作用下饱和黏土的各向异性、刚度、强度软化及塑性变形累积等特征。针对等压固结 和偏压固结 的饱和高岭黏土的不排水三轴试验进行模拟,采用不同的应变增量步长进行计算,并与试验数据对比,结果表明,修正隐式回映算法应用于该类边界面模型的合理性、积分格式的精确性和稳定性;另外,结合有限元软件自动时间步长的增量迭代解法,对饱和黏土应力控制的不排水动三轴试验进行预测,结果表明,修正的适用于该边界面的塑性模型隐式回映算法可以得到比较合理的数值分析结果,能够反映饱和黏土的循环刚度的退化和强度的弱化等动力特性。 相似文献
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In this paper, there is presented an elastoplastic constitutive model to predict sandy soils behavior under monotonic and cyclic loadings. This model is based on an existing model (Cambou‐Jafari‐Sidoroff) that takes into account deviatoric and isotropic mechanisms of plasticity. The flow rule used in the deviatoric mechanism is non‐associated and a mixed hardening law controls the evolution of the yield surface. In this research the critical state surface and history surface, which separates the virgin and cyclic states in stress space, are defined. Kinematic hardening modulus and stress–dilatancy law for monotonic and cyclic loadings are effectively modified. With taking hardening modulus as a function of deviatoric and volumetric plastic strain and with defining the history surface and stress reversal, the model has the ability to predict the sandy soils' behavior. All of the model parameters have clear physical meanings and can be determined from usual laboratory tests. In order to validate the model, the results of homogeneous tests on Hostun and Toyoura sands are used. The results of validation show a good capability of the proposed model. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
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This article presents an equivalent stress approach that can be used in many elastoplastic constitutive models for unsaturated soils. The use of the equivalent stress leads to a modified yield locus that is independent of the suction. In addition, the equivalent stress becomes the major stress variable, with suction required only as an additional variable in calculations. The model on the basis of equivalent stress predicts exactly the same soil behaviour, with the sole difference being the use of equivalent stress instead of original stress variables. This article also presents the equivalent stress formulations of several constitutive models for unsaturated soils, including the Barcelona Basic Model. The predictions from these models remain unchanged, with the only difference being in their implementation. Finally, the equivalent stress approach and the net stress approach are compared for the Barcelona Basic Model. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
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冻融循环对不同塑性指数路基土弹性模量的影响研究 总被引：1，自引：0，他引：1
在季节性冻土区,不同状态（塑性指数,围压等）路基土在通车运营期间经历冻融循环后,其物理力学性质会发生变化,但目前针对冻融循环对其的影响研究还不够广泛和深入,不能给季冻区路基设计提供准确的数据支持,故路基设计时并未将这种变化考虑其中,只取设计前期所采土样的试验值。取季节性冻土区3种不同塑性指数路基土,在室内采用最佳含水率制备成最大压实度试样,在封闭条件下经历0～7次完整冻融循环过程后,进行不同围压下的三轴压缩试验,在偏应力-应变曲线中通过直线段应力-应变关系得到弹性模量数据,结果表明,同种土相同冻融循环次数下,弹性模量随围压增加而增加;相同围压下弹性模量随冻融循环次数基本呈下降趋势。采用指数函数对所有弹性模量数据进行多元非线性拟合,建立不同塑性路基土弹性模量与围压及冻融循环次数的关系,拟合效果理想,可为缺乏相关资料的季冻区路基设计提供参考。 相似文献
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Giuseppe Buscarnera 《Acta Geotechnica》2014,9(2):313-327
The paper investigates the mathematical structure of plasticity models for unsaturated soils and provides a strategy to capture the loss of uniqueness of the incremental solution upon loading and/or wetting paths. To derive bifurcation conditions in simple analytical form, the analysis is restricted to isotropic stress states. This choice has allowed the inspection of the most common classes of constitutive models through a unified notation, as well as the study of different forms of coupling between plasticity and state of saturation. It is shown that, similar to saturated soil plasticity, the loss of admissibility of the plastic solution is governed by critical values of the hardening modulus. At variance with the classical case, however, these moduli can be positive even if the plastic flow rule is associated (bifurcation in the hardening regime). The paper shows that such non-trivial features derive from hydro-mechanical coupling, i.e. they depend on the approach used to reproduce suction effects and evolving retention properties. In other words, although the problem of loss of uniqueness affects all classes of plasticity models for unsaturated soils, different constitutive assumptions may not have the same outcome in terms of bifurcation potential. As a result, new concepts are introduced to compare the mathematical robustness of the different constitutive approaches, as well as to interpret their predictions in the light of precise bifurcation criteria. 相似文献
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A rigorous semi‐analytical solution for undrained cylindrical cavity expansion in critical state soils 
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下载免费PDF全文 Apostolos Vrakas 《国际地质力学数值与分析法杂志》2016,40(15):2137-2160
This paper presents a generalized, rigorous and simple large strain solution for the undrained expansion of a vertical cylindrical cavity in critical state soils using a rate‐based plasticity formulation: the initial stress field is taken as anisotropic, that is with horizontal stresses that differ from the vertical stress, and the soil is assumed to satisfy any two‐invariant constitutive model from the critical state (Cam‐clay) family; no simplifying assumption is made during the mathematical derivation; calculating the effective stresses around the cavity requires the solution of a nonlinear equation by means of the Newton–Raphson method in combination with quadrature. Cavity expansion curves and stress distributions in the soil are then presented for different critical state models (including the modified Cam‐clay model). The solution derived can be useful for estimating the instantaneous response of saturated low‐permeability soils around piles and self‐boring pressuremeters and can serve as trustworthy benchmark for numerical analysis codes. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
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In this paper, a simple bounding surface plasticity model is used to reproduce the yielding and stress–strain behavior of the structured soft clay found at Shanghai of China. A series of undrained triaxial tests and drained stress probe tests under isotropic and anisotropic consolidation modes were performed on undisturbed samples of Shanghai soft clay to study the yielding characteristics. The degradation of the clay structure is modeled with an internal variable that allows the size of the bounding surface to decay with accumulated plastic strain. An anisotropic tensor and rotational hardening law are introduced to reflect the initial anisotropy and the evolution of anisotropy. Combined with the isotropic hardening rule, the rotational hardening rule and the degradation law are incorporated into the bounding surface formulation with an associated flow rule. Validity of the model is verified by the undrained isotropic and anisotropic triaxial test and drained stress probe test results for Shanghai soft clay. The effects of stress anisotropy and loss of structure are well captured by the model. 相似文献
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The Barcelona basic model (BBM) successfully explained many key features of unsaturated soils and received extensive acceptance. It is also one of the few elastoplastic constitutive models for unsaturated soils that have been implemented within finite element codes and applied to the analysis of real boundary value problems. The BBM was proposed in incremental forms according to theories of soil plasticity in which individual aspects of the isotropic virgin behavior are controlled by multiple parameters, whereas at the same time, a single parameter controls more than one aspect of soil behavior. Although a variety of methods have been recently developed for calibrating model parameters for elastoplastic soil models, at present, there are no well‐established, simple, and objective methods for selecting parameter values in the BBM from laboratory tests. This has been one of the major obstacles to the dissemination of this constitutive model beyond the research context. This article presents an optimization approach especially developed for simple and objective identification of material parameters in the BBM. This is achieved by combining a modified state surface approach, recently proposed to model the elastoplastic behavior of unsaturated soils under isotropic stress conditions, with the Newton or quasi‐Newton method to simultaneously determine the five parameters governing isotropic virgin behavior in the BBM. The comparison between results using the proposed method and an existing method for the same laboratory tests was discussed from which the simplicity and objectivity of the proposed method were evaluated. Copyright © 2012 John Wiley & Sons, Ltd. 相似文献
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Yannis F. Dafalias Majid T. Manzari Achilleas G. Papadimitriou 《国际地质力学数值与分析法杂志》2006,30(12):1231-1257
SANICLAY is a new simple anisotropic clay plasticity model that builds on a modification of an earlier model with an associated flow rule, in order to include simulations of softening response under undrained compression following Ko consolidation. Non‐associativity is introduced by adopting a yield surface different than the plastic potential surface. Besides, the isotropic hardening of the yield surface both surfaces evolve according to a combined distortional and rotational hardening rule, simulating the evolving anisotropy. Although built on the general premises of critical state soil mechanics, the model induces a critical state line in the void ratio–mean effective stress space, which is a function of anisotropy. To ease interpretation, the model formulation is presented firstly in the triaxial stress space and subsequently, its multiaxial generalization is developed systematically, in a form appropriate for implementation in numerical codes. The SANICLAY is shown to provide successful simulation of both undrained and drained rate‐independent behaviour of normally consolidated sensitive clays, and to a satisfactory degree of accuracy of overconsolidated clays. The new model requires merely three constants more than those of the modified Cam clay model, all of which are easily calibrated from well‐established laboratory tests following a meticulously presented procedure. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献